1. Field of the Invention
This invention relates to integrated circuit fabrication and, more particularly, to methods for forming metallization structures.
2. Description of the Related Art
The information described below is not admitted to be prior art by virtue of its inclusion in this Background section.
An integrated circuit includes numerous active and passive devices arranged upon and within a single substrate. In order to implement desired circuit functions, select devices or components of the integrated circuit must be interconnected. Metallization structures are often used to interconnect integrated circuit components. Metallization structures may be generally subdivided into two categories: laterally extending interconnect lines and vertically extending contacts or plugs. Interconnects are relatively thin lines of conductive material that largely extend parallel to the underlying devices. As the name implies, contacts are the metallization structures that actually contact the devices of the integrated circuit. Plugs mostly extend vertically between metallization levels.
Within each level of interconnect, metallization structures are separated from other structures on underlying or overlying levels, and from structures within the same metallization level, by dielectric materials. The dielectric materials prevent unwanted communication between separated metallization structures. In large part because of the difficulty in etching many metallization materials, metallization structures are often formed by first depositing the dielectric material which will separate the metallization structures and then patterning cavities in the dielectric material (i.e., metallization cavities) for the metallization structures. The metallization cavities patterned for interconnect structures are typically called trenches, and the metallization cavities patterned for plugs are typically called vias. Once the cavities are formed, metals can then be deposited in the cavities to form metallization structures. If necessary, the deposited metal can be planarized, a process that often involves chemical-mechanical polishing (CMP).
Despite the name, metallization structures are not required to actually be metals, but may instead be fabricated from any material sufficiently conductive to transmit an electrical signal (e.g., doped polysilicon, metal silicides, refractory metal nitrides). For metallization structures above the level of local interconnect, though, metals are the predominant metallization materials, and one of the most common metallization materials is aluminum. Aluminum is desirable as a metallization material because of, among other things, its relatively low resistance and good current-carrying density.
Aluminum is usually deposited using physical vapor deposition (PVD). PVD processing may also be known as sputter deposition, or sputtering. In general, sputter deposition may be considered any deposition process in which a material is deposited by sputtering the material from, e.g., a target composed of the material to be deposited. A typical method for sputtering a metal onto a substrate includes introducing an inert gas into a deposition chamber and forming a plasma that ionizes the inert gas by applying a potential between the substrate and the target. The ionized inert gas atoms are then attracted toward the target, and collide with the target with such force that atoms of the target are sputtered off. The sputtered atoms may then deposit on the substrate.
Sputtering can be used to deposit any variety of materials, including conductors, non-conductors, and high melting point compounds. Sputtering is advantageous because it may provide for good step coverage and accurate transfer of material composition from the target to the deposited metal. This last feature is particularly helpful when depositing alloys.
One process for forming a metallization structure incorporating aluminum involves first sputter depositing a titanium wetting layer into the cavity in which the metallization structure will be contained. The titanium wetting layer lines the sidewalls and base of the cavity. A bulk metal layer of aluminum is then sputter deposited onto the wetting layer to fill the cavity. The titanium wetting layer helps to minimize or avoid agglomeration of the aluminum layer and provides for continuous metal coverage along the sidewalls and bottom of a cavity. In general, an effective wetting layer allows a subsequent bulk metal layer to be deposited more smoothly, and thus with higher quality.
Wetting layers are often deposited by standard sputtering processes, which may be considered to be that group of sputtering processes that do not impart any significant degree of directionality to the sputtered atoms. Standard sputtering processes thus allow sputtered atoms to contact the deposition surface at a variety of angles ranging from almost parallel to perpendicular. Standard sputtering processes, however, are unable to suitably deposit a wetting layer in small (i.e., narrow absolute width), high aspect ratio (i.e., cavity depth divided by the cavity width) cavities. The ability of a standard sputtering process to deposit an effective wetting layer is greatly affected by the width and aspect ratio of the cavity into which deposition is to occur. Generally speaking, the smaller the opening of the cavity, the less likely that high impact angle atoms will actually enter the cavity. So as aspect ratios increase, and as cavity openings become more narrow, the high quantity of high impact angle (e.g., those atoms having impact angles further away from perpendicular) atoms deposited in standard sputtering processes only increases the difficulty these processes often have in depositing effective wetting layers.
In an attempt to resolve this problem, many processes have implemented collimated sputtering processes when depositing a wetting layer. Generally speaking, collimated sputtering processes use a collimator arranged between the target and the substrate to block high impact angle atoms while allowing lower impact angle atoms (e.g., those atoms having impact angles closer to perpendicular) to pass through. As a result, collimated sputtering processes can be used to deposit adequate wetting layers in higher aspect ratio cavities than is possible with standard sputtering processes.
Collimated sputtering processes, however, are limited in that they only filter out high impact angle atoms, and as such do not impart significant directionality to the sputtered atoms that pass through the collimator. While providing improved performance over standard sputtering processes, collimated sputtering processes may also be unable to deposit an adequate wetting layer in narrow cavities having relatively large aspect ratios. As aspect ratios continue to increase and cavity openings continue to narrow, an adequate wetting layer cannot be formed using standard sputtering or collimated sputtering processes. Unfortunately, if an adequate wetting layer cannot be formed, a bulk metal layer (e.g., one composed of aluminum) deposited in the cavity may not have the desired quality. Consequently, the ability of such a metallization structure to transmit electrical signals may be impaired, or even destroyed.
Therefore, it would be desirable to develop an improved metallization structure and method for forming such a structure. The desired structure and method should be one that embodies an effective wetting layer even in cavities having high aspect ratios.